Conventionally, there have been disclosed an image pickup apparatus which obtains a clear image under a low illuminance condition by taking images of a target object by use of an electron multiplying-charge coupled device (EM-CCD) and a method of making the sensitivity of an output image uniform by controlling the EM-CCD to be cooled at a predetermined level (see, e.g., Japanese Patent Application Publication No. 2007-318735).
A plurality of image pickup apparatuses using such EM-CCDs are installed and images output therefrom are recorded or displayed on monitors to be monitored by users. However, as shown in FIG. 4, variations exist in the electron multiplication properties of the EM-CCDs. Accordingly, even if the sensitivities of the image pickup apparatuses are set as an identical value, sensitivity differences may occur in images output therefrom due to such variations, especially when an amplification performed by an amplifier unit exceeds an upper limit (or a predetermined upper limit). Therefore, for example, when the images with the sensitivity differences are displayed on monitors, it sometimes becomes difficult to distinguish and compare the images and thus monitor important matters such as an intruder and the like, which may cause such important matters to be missed.